Agricultural waste materials such as rice hulls, rice straw, wheat chaff and straw have a potential useful fuel value which is difficult to convert into useful energy or a useful product. In fact, it now represents a substantial problem to the industry to dispose of the large quantity of unuseable waste materials produced in agriculture.
It has also been recognized that certain agrecultural organic materials are high in biogenetic silica, that is, silica occurring within the cell structure. Principally, rice hulls, rice stalks, equisetum, (a common weed popularly known as horsetails) and certain palm leaves, particularly palmyra palm, have varying amounts of silica in the cell structure. In addition, certain bamboo stems are known to contain relatively large amounts of silica and wheat straw contain from 2 to 3 percent silica in the cell structure. For example, most rice hulls are generally found to contain approximately 20% silica while rice straw may have approximately 181/2% silica. Certain California horsetail weeds are known to have about 20 to 25% silica.
The disposition of rice hulls and rice straw has become a substantial problem to the rice growing industry inasmuch as they apparently can be disposed of only by burning or burying. Dump space for burying the material has become scarce in the last few years and the incineration of the silica bearing hulls and straw in open fields generally creates undesirable atmospheric pollutants.
It has been known, of course, that silica, along with calcium oxides, is a component of portland cement, primarily occurring as complex calcium silicates. However, the silica produced by the incineration of silica containing organic agricultural materials can be used as a cement component only to the extent that it replaces sand or shale, because the incineration of the hulls inevitably leads to the production of ash containing crystalline forms of silica.
The phase diagram of silicon dioxide indicates that a transition from the amorphous, non-crystalline form of material to the crystalline forms known as tridymite and crystobalite takes place at very high temperatures when the silica is in pure state. Thus, with pure silica in the amorphous form, it is theoretically necessary to raise the temperatures above 2000.degree.F to effectuate the transformation. However, the incineration of rice hulls, even at temperatures substantially below 2000.degree.F, has always lead to the formation of crystalline varieties of silica, because the transition temperature from amorphous to crystalline is reduced substantially by the presence of other components of the original rice hulls.
Prior attempts to use crystalline silica as an active component of portland cement have always involved heat treating mixtures of limestone and siliceous shales or clays at temperatures in excess of 2600.degree.F. The same objective can also be accomplished by application of mechanical energy. Experimentally, it has been shown that attrition grinding of crystalline quartz can activate the silica by rupturing the chemical bonds at the surface. In addition to attrition grinding, vibromilled sand-lime mixtures which are attrition ground, have been reported to have acquired binding properties, and thus treated lime-sand mixtures have been used as hydraulic cements.
These processes, except for the commercial process of heating a siliceous raw material with limestone, appear to be commercially unattractive because of the excessive mechanical energy necessary in the vibromilling and attrition grinding processes.
It has recently been discovered that a highly useful amorphous form of silica may be produced by the careful, controlled incineration of rice hulls at relatively low temperatures and oxidizing atmospheres. In the original discovery, rice hulls were incinerated in a relatively oxidizing atmosphere in an electric furnace in small quantities by gradually raising the temperature of the rice hulls. It was observed that smoke began to generate from the rice hulls at about 400.degree.F and that as the incineration temperature increased up to about 1200.degree.F, it was possible after long exposure to heat to produce a product containing almost completely amorphous silica with a relatively low loss on ignition content (indicative of residual carbon in the mass).
It has also been found that on exposure to temperatures above approximately 1300.degree.F, a transition to the crystalline forms of silica takes place, and the extent of this transition is dependent upon the length of time of exposure to these elevated temperatures.
It has been discovered that this amorphous form of silica is highly reactive in nature and has utility in the preparation of novel cement compositions similar in property to portland cement.
The laboratory method used to produce the amorphous silica was not amenable to the large scale disposition of rice hull, or in fact, the relatively large scale preparation of the amorphous product thereby produced.
It is to be noted that according to the phase diagram of silicon dioxide, when the compound is in the relatively pure state, amorphous silica must be heated to temperatures in excess of about 2600.degree.F before there is a transition to the tridymite or crystobalite forms of silica. However, in the impure form of silica found in rice hulls, it has been experimentally observed that the transition from the amorphous form to the crystalline tridymite or crystobalite form occurs at a much lower temperature, due to the impurities present. In the past it has been common practice, solely for the purpose of disposing of the rice hull waste, because of its large bulk density in the unburned condition, to simply incinerate the rice hulls in the most rapid expeditious manner. This has been accomplished in several ways. Bulk burning of the rice hulls has been resorted to in large piles of rice hulls in which the temperatures vary considerably depending upon the location of the hull in the pile. In other methods, special furnaces have been designed to incinerate the rice at extremely high temperatures without regard to the form of silica produced by the incineration. In all of these methods, crystalline silica has been the result and this crystalline silica is of little commercial utility. Thus, in the usual incineration of rice hulls at 1800.degree.F, any prolonged exposure to the 1800.degree.F temperature causes an almost complete transformation from the amorphous form of silica to crystalline tridymite or crystobalite form.
In attempting to overcome the disadvantages of prolonged slow oxidation in the incineration of rice hulls to produce amorphous silica, it has been found that there is a process which will not only form silica having a high degree of amorphous silicon dioxide but having a variable range of residual organic material which has a high degree of utility as a cement component.